Author Topic: Pianet CH Radio Circuit and Pickups  (Read 1553 times)

Offline groovemonkey73

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Pianet CH Radio Circuit and Pickups
« on: June 28, 2020, 01:33:34 PM »
Hey all!
Just picked up this rather charming Hohner Pianet CH from the early to mid 60s. I got it knowing there was an output issue, with the rather curious little radio circuit being the possible culprit. The Pianet powers up, the trem seems to work as the light bulbs lights up and flashes when engaged, and when plugged in, the hum pulsates at the same rate as the lights flashing. The output hum sounds much like a ground hum.

I've had the radio circuit out, noticed the little coil wasn't soldered to one of the posts, and hoped thats all it was, but unfort wasn't.

Also, is there any quick/easy way of knowing/testing the pickup(s)? There doesn't seem to be a lot of info/wiring diagrams out there! - Anyone much experience with this era of Pianets?!
1979 Mk1 Rhodes 73
1964 Hohner Pianet CH
Nord Stage 2 Compact
Hammond XK3c
1915 John Broadwood & Sons Upright

Offline sean

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Re: Pianet CH Radio Circuit and Pickups
« Reply #1 on: June 28, 2020, 03:16:19 PM »

Offline groovemonkey73

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Re: Pianet CH Radio Circuit and Pickups
« Reply #2 on: June 29, 2020, 04:22:41 AM »
Thanks Sean, really appreciate that.
1979 Mk1 Rhodes 73
1964 Hohner Pianet CH
Nord Stage 2 Compact
Hammond XK3c
1915 John Broadwood & Sons Upright

Offline Jenzz

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Re: Pianet CH Radio Circuit and Pickups
« Reply #3 on: June 30, 2020, 01:32:24 PM »
Pianet CH Owners Manual incl. Schematic:

http://www.hohner-pianet.com/hohner_pianet/pianet_ch.html


Jenzz
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Offline theseacowexists

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Re: Pianet CH Radio Circuit and Pickups
« Reply #4 on: July 02, 2020, 01:37:31 PM »
Are you getting any signal, or just the hum?

Offline groovemonkey73

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Re: Pianet CH Radio Circuit and Pickups
« Reply #5 on: July 03, 2020, 10:45:53 AM »
Just hum at the moment!
1979 Mk1 Rhodes 73
1964 Hohner Pianet CH
Nord Stage 2 Compact
Hammond XK3c
1915 John Broadwood & Sons Upright

Offline theseacowexists

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Re: Pianet CH Radio Circuit and Pickups
« Reply #6 on: July 06, 2020, 06:28:06 AM »
When I got my (then dead) Cembalet CF (same circuitry as yours http://www.hohner-cembalet.com/hohner_cembalet/Cembalet_CF.html), I thought the same thing. I was stumped until I had a tech replace the electrolytic caps and the rectifier. That brought everything back. He also said that the schematic had the rectifier printed backwards - something I never would have caught.

If I remember correctly, the way this circuit works, you won't find any voltage on the pickup, as you would on the other models. There should also be continuity between the pickup and the reeds, which also confused the heck out of me, but is apparently also normal in this model.

The pads are in good shape, right?

Offline Jenzz

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Re: Pianet CH Radio Circuit and Pickups
« Reply #7 on: July 06, 2020, 11:02:33 AM »
Hi .-)

The circuit works with positive ground.

The Reeds and the Pickup are tied together via the output coil of the 'radio circuit'.

Jenzz
Rhodes tech in Germany
www.tasteundtechnik.de
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Offline groovemonkey73

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Re: Pianet CH Radio Circuit and Pickups
« Reply #8 on: July 20, 2020, 08:41:56 AM »
Thanks so much for your replies and input Jenzz and theseacowexists (great handle btw)!

I'm about to have another little play about with it, but was leaning towards replacing the caps and resistors in the radio circuit and then seeing what happens. Thanks for the info on the schematic - may make a little more sense no!
1979 Mk1 Rhodes 73
1964 Hohner Pianet CH
Nord Stage 2 Compact
Hammond XK3c
1915 John Broadwood & Sons Upright

Offline groovemonkey73

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Re: Pianet CH Radio Circuit and Pickups
« Reply #9 on: July 20, 2020, 08:54:32 AM »
To help me on ny quest however, could someone point me towards where yhis radio circuit marries upto on either the CH schematic and/or the cembalet schematic?

https://app.photobucket.com/u/butcher650/p/e05332fa-47d5-483f-a60e-7a613be49db1
1979 Mk1 Rhodes 73
1964 Hohner Pianet CH
Nord Stage 2 Compact
Hammond XK3c
1915 John Broadwood & Sons Upright

Offline theseacowexists

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Re: Pianet CH Radio Circuit and Pickups
« Reply #10 on: July 22, 2020, 10:09:45 AM »
Interesting - your radio circuit is on a separate board. Mine has all of the circuitry all on one board. I bet that having it in that metal case helps with the shielding, mine is prone to picking up other radio stations sometime!

Anyway, to answer your question about where this is on the schematic, it is in the upper left. The black thing with the red stripe is diode OA79, the three legged black thing is transistor AF101, the yellow thing is the radio transformer (squiggly lines on the schem). The transformer should have four leads which attach to the four lugs near it on the board.

I'm assuming that the black wire with the braid are the wires to the harp. The bare braided wire should go to the reeds ("zunge") and the black one should go to the pickup ("elektrode"). Make sure there is continuity between the ends of the wires.

What about your sticky pad situation? Any originals would be completely disintegrated by now, meaning that playing any key would not generate any sound. If that's the case, pluck any of the reeds with a wooden dowel or plastic pen and see if that generates any sound. Don't use your fingers, there's a risk of electric shock!

Offline groovemonkey73

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Re: Pianet CH Radio Circuit and Pickups
« Reply #11 on: July 23, 2020, 04:30:34 AM »
Yeah, its completely separate - and in that metal box. Not having had it working properly, I hadn't thought about the shielding properties as it was a pain in the neck to get into as it was soldered shut. I was thinking of mounting the board without it, but maybe not now!

Thank you so much - I see what you mean. I stared at it and the board the other day for what seemed like hours, and I thought it was top left because of the coil parts - and then I slowly started tracing the components. Those black wires go to the left side of the other board, which I assume must be like you say. I'll get it going - massive learning curve for me reading the schematic and trying to translate it to the parts in front of me. I've ordered some replacement resistors, need to try and find some caps etc now. I actually re-wound that little transformer part when I first got it open as noticed the wires not attached. They show continuity on the board, so I know that bit is OK. - I was hoping that would fix it, but I think there are other gremlins in this radio board. The vibrato I THINK works as the Pianet powers up, and the light on the trem board blinks when you engage the switch, but whether it will when I've sorted the radio circuit and got it outputting, we'll see!

Some of the sticky pads are OK, some aren't - I think they've been replaced at SOME point in it's life, but they all need replacing again for sure. I want to it get it outputting first - then I'll address the other bits, but pads definitely. It needs completely stripping and cosmetically restoring really - but like my Rhodes - that will probably be the 25th of never! As long as I can get it working and working well, the cosmetics can wait a bit!
1979 Mk1 Rhodes 73
1964 Hohner Pianet CH
Nord Stage 2 Compact
Hammond XK3c
1915 John Broadwood & Sons Upright

Offline theseacowexists

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Re: Pianet CH Radio Circuit and Pickups
« Reply #12 on: July 23, 2020, 07:18:35 AM »
Have you replaced the electrolytic capacitors yet? That should be first order of business since they're over 50 years old and have become leaky. Looks like there's two 1000uf, one 50uf (use 47uf), one 25uf, one 5uf (use 4.7uf), and one 100uf. Use the same or greater voltage rating, and be mindful of their polarity. Discharge the only ones by clipping a lead from the positive end to the chassis/ground, unless ya want a nice lil jolt! Which I'm sure ya don't...

Offline groovemonkey73

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Re: Pianet CH Radio Circuit and Pickups
« Reply #13 on: August 03, 2020, 01:47:12 AM »
I haven't no  - I think they've already recently been done (by the guy before me who tried to get it going)! They look pretty new, and because all the stuff in there works I'm hoping they're OK. What do you think?
1979 Mk1 Rhodes 73
1964 Hohner Pianet CH
Nord Stage 2 Compact
Hammond XK3c
1915 John Broadwood & Sons Upright

Offline groovemonkey73

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1979 Mk1 Rhodes 73
1964 Hohner Pianet CH
Nord Stage 2 Compact
Hammond XK3c
1915 John Broadwood & Sons Upright

Offline theseacowexists

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Re: Pianet CH Radio Circuit and Pickups
« Reply #15 on: August 27, 2020, 01:59:51 PM »
Interesting, even the power supply is on a separate board. Totally different than my Cembalet!

Anyway, those caps look new. The sticky pads appear to be the clavinet.com replacements too.

The wires are attached to the pickup and reed, right?

Just want to rule out anything obvious. Make sure ALL the wires are attached where they should be.All the way down to the output jacks.  Reflow the solder at any suspect joints.

Offline Bill Ebrill

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Re: Pianet CH Radio Circuit and Pickups
« Reply #16 on: November 01, 2021, 05:04:25 AM »
I have a Pianet CH with a Chambalet CF printed circuit board and I am attempting to repair it. It uses a radio circuit as the pickup.

This is a single board which contains everything except the transformer. The person who repaired it before me used a 'bell' transformer and replaced all the electrolytics.There is lots of induced hum so I started with a 12v battery but the noise still outweighs the notes. The radio pickup coil got damaged along the way but it does function.

The radio circuit is a challenge. I though I knew a lot about radio but this has stretched me. I have decided to build a replica circuit on the bench to understand it more completely.

My assessment so far (subject to further enlightenment) is that the AF101 is a Colpitts oscillator running at about 2.4Mhz. This is air coupled to the second keyboard coil, on the same coil former. This second coil resonates at or about the same frequency as the oscillator. The second coil is tuned with the capacitance of the keyboard which is about 110pF. This is fixed.

Minute changes in capacitance, as the reeds/tines vibrate, cause the resonant frequency of the second coil to change frequency. This changes the 'slope' of the single diode 'slope detector' and the recovered changes are detected as audio frequencies, i.e. the notes or the reed.

This is like reverse frequency modulation (FM) detection whereas, instead of modulating the oscillator, the detection coil which determines the single diode slope detector is modulated.

An oscilloscope is required to see what is happening. I have detected these signals but the noise and hum makes it impossible to use.

I will post further updates as I make progress. My intention is to build the oscillator, coil and detector circuit. if I get it working noise free I will use it as a comparator to complete the repair of the Pianet.

I would appreciate any insights. The trickiest part so far is constructing a replica radio pickup coil.

Offline theseacowexists

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Re: Pianet CH Radio Circuit and Pickups
« Reply #17 on: November 04, 2021, 07:37:54 AM »
http://hohner-pianet.com/hohner_pianet/pianet_ch.html

There are service manual scans on this page, and the English section gives a description of how the radio circuit works. Not sure if this will help you, but there it is.